Heat the butter in a large saucepan. Add the onion and a pinch of salt, and cook gently for 5 mins. Tip in the parsnips, turn up the heat and cook for 8-10 mins, stirring every so often, until the parsnips are starting to brown and caramelise.
Add the garlic and sage, and mix through. Tip in the barley and stir to coat well. Pour in the stock, bring to the boil, then turn down to a simmer and cook for 35-40 mins, or until nearly all the liquid has been absorbed and the pearl barley is tender but still has a bite. You may need to add a little extra boiling water.
Take off the heat, top with the Parmesan and a knob of butter, then leave to melt. Give the risotto a good stir, then spoon into dishes. Top with more sage, Parmesan and some black pepper.